ATC technologies are evolving to include other components than primary and secondary surveillance radars, but I believe these two pieces of equipment are still the backbone of ATC. Primary radar (PSR, 2.8 GHz) works usually on higher frequencies than secondary radar (SSR, 1.030 GHz).
Primary radar uses the natural echo reflected by the target to determine its angular position and range, while secondary radar uses the cooperative answer provided by the target transponder. The answer is equivalent to a natural echo, but is delayed by a known time, and is more powerful. It can also contain additional information such as level.
I read that SSR is more accurate in position determination than PSR. Indeed SSR doesn't work without the support of the airborne transponder, and the target can disappear from SSR display when the transponder has failed or has been disconnected.
In appearance SSR + transponder can provide information to ATC without the need for a primary radar support. Is it true, if not why?
SSR can technically receive a natural echo on 1.030 GHz before the SSR reply which is sent on 1.090 GHz, is this signal used?
Do ATC systems usually display PSR data for controllers continuously or only on demand (e.g. when a target transponder doesn't answer SSR interrogator)?